1. Field of the Invention
This invention relates to a color liquid crystal display, more particularly, to a color liquid crystal display performing color display using birefringence of a liquid crystal device, without using color filters.
2. Description of the Related Art
There are two kinds of color liquid crystal displays, that is, one embodies a color filter and the other performs color display using, birefringence of a liquid crystal without using a color filter.
In a liquid crystal display having a color filter therein, since a pixel is composed of three elements of R, G, B, the amount of transmitted light is reduced to around one third and the liquid crystal display usually requires a small fluorescent light as a back light, the liquid crystal display with a color filter is not suitable for a reflection-type color liquid crystal display.
On the other hand, a liquid crystal display which performs color display utilizing birefringence of a liquid crystal is suitable for the reflection-type color liquid crystal display, because color displaying can be performed with one pixel only by changing the voltage applied to a liquid crystal device.
As the color display utilizing the birefringence of the liquid crystal, the followings are known:
(1) A color liquid crystal display composed only of a liquid crystal device and a pair of polarizing films.
(2) A color liquid crystal display composed of a liquid crystal device, a retardation film and a pair of polarizing films.
(3) A color liquid crystal display composed of a liquid crystal device, a twisted retardation film and a pair of polarizing films.
As a liquid crystal device for a color liquid crystal display, a homogeneous liquid crystal device having a twisted angle of zero degrees, a TN (twisted nematic) liquid crystal device having a twisted angle of 90 degrees, and an STN (super twisted nematic) liquid crystal device having a twisted angle between 180 to 270 degrees have been developed.
A conventional example of a color liquid crystal display using a twisted retardation film and adopting an STN liquid crystal device as a liquid crystal device will be explained with reference to FIG. 16 to FIG. 18.
FIG. 18 is a schematic sectional view of the above-described color liquid crystal display, FIG. 16 is a plan view showing a relation between the absorption axes of lower polarizing films and the molecular alignment direction in the liquid crystal, obtained when FIG. 18 is viewed from the upper polarizing film 9 side, and FIG. 17 is also a plan view showing a relation between the absorption axes of the upper polarizing film and the twisted retardation film.
Such a color liquid crystal display is disclosed, for instance, in Japanese Patent Laid-open No. Hei 7-5457.
In the color liquid crystal display, as shown in FIG. 18, a liquid crystal device 20 is formed by holding a nematic liquid crystal 7 in a twisted alignment between a pair of substrates composed of a first substrate 1 which is formed with an alignment layer 3 and a first electrode 2 made of ITO (Indium Tin Oxide) and a second substrate 4 which is formed with an alignment layer 6 and a second electrode 5 made of ITO.
Further, a pair of polarizing films, that is, a lower polarizing film 8 and an upper polarizing film 9, are disposed holding the above-described liquid crystal device 20 thereinbetween, a twisted retardation film 10 is disposed between the liquid crystal device 20 and the upper polarizing film 9, and a reflecting plate 11 is disposed outside of the lower polarizing film 8.
Absorption axes (or transmission axes) of the pair of polarizing films 8 and 9 are disposed in parallel. Here, the twisted angle of the liquid crystal device 20 is 250 degrees. The absorption axis 8a of the lower polarizing film 8 shown by a broken line with arrows in FIG. 16 intersects with the lower molecular alignment direction 7a in the liquid crystal, that is the alignment direction of the liquid crystal in the first substrate 1, at an angle of 45 degrees. The absorption axis 9a of the upper polarizing film 9 shown by a solid line with arrows in FIG. 17 is disposed to intersect with the upper molecular alignment direction 10b in the twisted retardation film 10 at an angle of 45 degrees.
Incidentally, the reference numeral 7b in FIG. 16 shows the upper molecular alignment direction in the liquid crystal, that is the alignment direction in the liquid crystal of the second substrate 4, and the reference numeral 10a in FIG. 17 shows the lower molecular alignment direction in the twisted retardation film 10.
The .DELTA.nd value of the liquid crystal device 20 expressed by the product of a difference of the birefringence .DELTA.n of the nematic liquid crystal 7 and a cell gap d, that is a space between the first substrate 1 and the second substrate 2, is 843 nm. A twisted angle of the twisted retardation film 10 is 250 degrees in the reverse direction of the twisted angle of the liquid crystal device 20. The .DELTA.nd value of the twisted retardation film, which is expressed by the product of a difference .DELTA.n of the birefringence of the twisted retardation film 10 and the thickness d, is also 843 nm.
As shown in FIG. 17, since the absorption axis 9a of the upper polarizing film 9 is disposed to intersect with the upper molecular alignment direction 10b in the twisted retardation film 10 at an angle of 45 degrees, linearly polarized light incident from the upper polarizing film 9 becomes elliptic polarized after passing through the twisted retardation film 10.
However, since the upper molecular alignment direction 7b of the liquid crystal in the liquid crystal device 20 deviates from the lower molecular alignment direction 10a in the twisted retardation film by 90 degrees, the elliptic polarized light generated at the liquid crystal device 20 and the twisted retardation film 10 is placed back into its original state of linearly polarized light and reaches the lower polarizing film 8. Since the absorption axis 8a of the lower polarizing film 8 is parallel to the absorption axis 9a of the upper polarizing film 9, a white display is shown.
When voltage is applied between the first electrode 2 and the second electrode 5, the liquid crystal molecules 7 are activated and the apparent .DELTA.nd value of the liquid crystal device 20 is decreased. Accordingly, the elliptic polarized light generated at the twisted retardation film 10 cannot be completely canceled by the liquid crystal device 20, and reaches the lower polarizing film 8 without changing its elliptical polarization state. Accordingly, light beams having a specific wavelength penetrate therethrough and generate several colors.
The light that has passed through the lower polarizing film 8 is reflected by a reflecting plate 11 and emits upwards again after passing through the lower polarizing film 8, the liquid crystal device 20, the twisted retardation film 10 and the upper polarizing film 9. Thus, a reflection-type color display can be obtained. That is, it can display in white when no voltage is applied, but according to a voltage increase, it can display in several colors such as yellow, violet, red and so on.
Next, a conventional color liquid crystal display using a retardation film and adopting an STN liquid crystal device as a liquid crystal device will be explained with reference to FIG. 19 to FIG. 21. FIG. 21 is a schematic sectional view of the color liquid crystal display, FIG. 19 is a plan view showing a relation between absorption axes of a lower polarizing film and the molecular alignment direction in the liquid crystal obtained when FIG. 21 is viewed from the upper polarizing film 9 side, and FIG. 20 is also a plan view showing a relation between absorption axes of an upper polarizing film and the phase delay axis of each retardation film.
Such a color liquid crystal display is disclosed, for instance, in Japanese Patent Laid-open No. Hei 8-15691.
As shown in FIG. 21 the color liquid crystal display is comprised of: a liquid crystal device 21 (the twisted angle is the same as that of the liquid crystal device 20 in FIG. 18 but the cell gap is different); a pair of polarizing films, that is, a lower polarizing film 8 and an upper polarizing film 9, disposed holding the liquid crystal device 21 thereinbetween; a first retardation film 15 and a second retardation film 16 disposed between the liquid crystal device 21 and the upper polarizing film 9; and a reflecting plate 11 disposed outside of the lower polarizing film 8.
The absorption axes (or transmission axis) of the pair of polarizing films 8 and 9 are disposed intersecting at almost right angles. Here, the twisted angle of the liquid crystal device 21 is 250 degrees. The absorption axis 8a of the lower polarizing film 8 shown by a broken line with arrows in FIG. 19 intersects with the lower molecular alignment direction 7a in the liquid crystal, that is an alignment direction of the liquid crystal in the first substrate 1, at an angle of 45 degrees. The phase delay axis 16a of the second retardation film 16 shown by a broken line in FIG. 20 is disposed to intersect with the upper molecular alignment direction 7b of the liquid crystal in the liquid crystal device 21 at an angle of 95 degrees, and the absorption axis 9a of the upper polarizing film 9 shown by a solid line with arrows in FIG. 20 is disposed to intersect with the phase delay axis 15a of the first retardation film 15 at an angle of 15 degrees.
The .DELTA.nd value of the above described liquid crystal device 21 is between 1530 nm and 1730 nm. The retardation value of the first retardation film 15 is 1600 nm and the retardation value of the second retardation film 16 is 1550 nm.
Since the absorption axis 9a of the upper polarizing film 9 and the phase delay axis 15a of the first retardation film 15 intersect with each other at an angle of 15 degrees, linearly polarized light incident from the upper polarizing film 9 becomes elliptic polarized after passing through the first retardation film 15 and the second retardation film 16.
However, since the upper molecular alignment direction 7b of the liquid crystal in the liquid crystal device 21 and the phase delay axis 16a of the second retardation film 16 deviate from each other by about 90 degrees, elliptic polarized light generated at the first retardation film 15 and the second retardation film 16 is returned to almost its original state as linearly polarized light to reach the lower polarizing film 8. Since the absorption axis 8a of the lower polarizing film 8 intersects with the absorption axis 9a of the upper polarizing film 9 at almost right angles, a black state is obtained.
When voltage is applied to the first electrode 2 and the second electrode 5, the molecules of the nematic liquid crystal 7 are activated and the apparent .DELTA.nd value of the liquid crystal is decreased. Accordingly, since the elliptic polarized state generated at the first retardation film 15 and the second retardation film 16 cannot be completely canceled by the liquid crystal 21 and reaches the lower polarizing film 8 without changing its elliptic polarized state, light beams having specific wavelengths transmit and generate a plurality of colors.
Since the beams passing through the lower polarizing film 8 are reflected by the reflecting plate 11, and again pass through the lower polarizing film 8, the liquid crystal device 21, the second retardation film 16, the first retardation film 15, and the upper polarizing film 9, and then emit upward, a reflection-type color display can be obtained. That is, it displays in black when no voltage is applied, but according to voltage increase, it displays in several colors such as white, red, blue, green and the like.
However, in an actual color liquid crystal display, since variations in every wavelength dependence of .DELTA.n in the birefringence of the liquid crystal material are different from the variations in every wavelength dependence of .DELTA.n in birefringence of the retardation film or the twisted retardation film, good background colors or tints cannot be obtained with a conventional arrangement of the polarizing films as described above.
With regard to a negative-type display for changing colors of letters while displaying the background color in black, the above described literature discloses that it is possible to dispose the absorption axis 9a (or transmission axis) of the upper polarizing film 9 and the absorption axis 8a (or transmission axis) of the lower polarizing film 8 in a manner that they intersect with each other at almost right angles. However, the literature does not disclose the best condition suitable for coloring the negative-type display, and a reflection-type color liquid crystal display which makes display possible in bright and good color saturation has not yet become practical.